The present invention relates to an apparatus and process for measuring and controlling the basis weight and composition of a coating applied to a sheet, or a filler mixed in the base sheet material, wherein the coating or filler includes at least 3 components of differing atomic weight.
For the purposes of simplicity and clarity, the following discussion relates to the use of x-ray analysis to determine the amount and composition of "ash" applied in or on a moving sheet of paper being manufactured on a papermaking machine. However, the invention is not limited to use with paper sheet, nor to an ash coating or filler material, nor to x-rays. The present invention may be used with other sheet materials, with other coating and filler materials, and with electromagnetic radiation of various energies.
Paper is generally made of three constituents: water, wood pulp fiber, and ash. "Ash" is defined as that portion of the paper which remains after complete combustion. In particular, ash may include various mineral components such as chalk (alternatively referred to as calcium carbonate or CaCO.sub.3), titanium dioxide (TiO.sub.2), and clay (a major component of clay is SiO.sub.2).
Because certain of the components of ash may be less expensive than other components and because the relative amounts of the ash components effect the physical properties of the finished paper product, such as the opacity and brightness of the paper, it is desirable to be able to measure the amounts of the various ash components during the papermaking process. Based upon these measurements, the total amount of ash added to the sheet and the relative amounts of the individual ash components are preferably controlled to optimize the desired paper characteristics and minimize cost.
Ash may include various other non-combustible materials in addition to or in place of calcium carbonate, titanium dioxide, and clay, such as, for example, iron oxide (FeO). Iron oxide is used to make reddish-brown colored paper. However, many paper manufacturers use clay, titanium dioxide and calcium carbonate for typical white paper sheet. Furthermore, clay is generally far cheaper than wood pulp fiber. Therefore, it is often important to maintain the clay content as high as reasonably possible, while still maintaining other physical characteristics of the paper within the desired specification limits.
The total amount of ash in paper and the composition of the ash can be controlled manually by setting the rates of flow of clay and other ash components as well as the flow of wood pulp fiber and water to the papermaking system. The resulting sheet is periodically sampled and burned in the laboratory to determine the composition and amount of ash in the sheet. When the ash content is determined in the laboratory, the paper is burned under predetermined conditions and the resulting ash is accurately weighed and chemically analyzed. The papermaking parameters can then be altered based upon the resulting measurements. However, this procedure of manual control suffers from the main disadvantage that it is time consuming. Thus, large quantities of paper which do not meet specifications may be manufactured while the laboratory tests are being conducted.
The relatively long time needed for laboratory ash measurements can be particularly troublesome when the ash is contained in a coating applied to the surface of the paper sheet, rather than being mixed into the sheet itself. In the former situation, a device known as a "coater" is used to apply the ash-containing coating material to one or both surfaces of the sheet. Because the coating material will usually be applied as an extremely thin layer, coaters require essentially continuous automatic monitoring and adjustment at numerous positions (called "slices") across the width of the sheet to maintain the coating within specification limits. Therefore, when manufacturing a coated sheet, a papermaker usually cannot afford to wait for laboratory ash measurements without running the risk of producing substantial amounts of coated paper which is outside specification limits, and therefore potentially unusable.
The prior art discloses a method of using x-ray analysis during the papermaking process to determine the total amount of ash or amounts of the individual ash components mixed into a moving sheet of paper. This is accomplished by directing one or more x-ray beams into the paper and detecting that portion of the beam or beams which is transmitted through the sheet.
The proportion of an x-ray beam which is transmitted though the sheet is called its transmittance, T, and is equal to the ratio of the beam intensity before and after the beam is transmitted through the sheet, that is: EQU T=I/Io, (1)
Where:
I=intensity of the beam after it is transmitted through the sheet; and
Io=intensity of the beam before it is transmitted through the sheet.
The transmittance of the beam through the sheet is defined by Beer's law, as follows: EQU T=e.sup.-u T.sup.W T. (2)
Where:
u.sub.T =effective mass absorption coefficient of all the different constituent materials forming the sheet; and
W.sub.T =total mass of these constituent materials.
Typically, when sheet materials are being measured, W.sub.T =is expressed as a "basis weight", that is, in units of mass per unit surface area of the sheet.
For sheet materials which include several constituents, such as a paper sheet including an ash-containing filler or coating, the exponent of equation (2) can be expanded, thus: EQU u.sub.T W.sub.T =u.sub.ash W.sub.ash +u.sub.fiber W.sub.fiber +u.sub.water W.sub.water. (3)
Where:
u.sub.ash =mass absorption coefficient of the ash;
W.sub.ash =basis weight of the ash;
u.sub.fiber =mass absorption coefficient of the fiber;
W.sub.fiber =basis weight of the fiber;
u.sub.water =mass absorption coefficient of the water; and
W.sub.water =basis weight of water.
The mass absorption coefficients for fiber and water are known. Further, paper making machines typically include sensors to determine both the water or moisture content and the basis weight of the paper sheet. Therefore, W.sub.water and W.sub.T are also known. The weight of the fiber can be expressed in terms of the basis weight of the sheet minus the weight of the ash and water in the sheet, as follows: EQU W.sub.fiber =W.sub.T - (W.sub.ash +W.sub.water) (4)
Therefore, only the ash term of equation (3), i.e., u.sub.ash W.sub.ash, is unknown.
For multi-component ash materials including clay, calcium carbonate and titanium dioxide, the ash term of equation 3 can be still further expanded, thus: EQU u.sub.ash W.sub.ask =u.sub.clay W.sub.clay +u.sub.Ca W.sub.Ca +u.sub.Ti W.sub.Ti. (5)
Where:
u.sub.clay =mass absorption coefficient of clay;
W.sub.clay =basis weight of clay;
u.sub.Ca =mass absorption coefficient of CaCO.sub.3 ;
W.sub.Ca =basis weight of CaCO.sub.3 ;
u.sub.Ti =mass absorption coefficient of TiO.sub.2 ; and
W.sub.Ti =basis weight of TiO.sub.2.
The mass absorption coefficients, u.sub.clay, u.sub.Ca, and u.sub.Ti, are known functions of the x-ray beam tube target voltage, the spectral width of the x-ray beam, and the spectral configuration of the x-ray beam, i.e., the magnitudes of the various energies contained within the x-ray beam spectrum. As a result, the only remaining unknowns in equation (5) are W.sub.clay, W.sub.Ca, and W.sub.Ti, that is, the basis weight of each of the individual ash components.
By measuring the transmittance through the ash-containing sheet of 3 x-ray beams having different and independent spectral configurations, it is possible to establish 3 independent equations, one for each beam in the form of equation (2) where, for beams 1, 2 and 3:
______________________________________ Beam 1: u.sub.ash.sbsb.1 W.sub.ash = u.sub.clay.sbsb.1 W.sub.clay + u.sub.Ca.sbsb.1 W.sub.Ca + u.sub.Ti.sbsb.1 W.sub.Ti (6) Beam 2: u.sub.ash.sbsb.2 W.sub.ash = u.sub.clay.sbsb.2 W.sub.clay + u.sub.Ca.sbsb.2 W.sub.Ca + u.sub.Ti.sbsb.2 W.sub.Ti (7) Beam 3: u.sub.ash.sbsb.3 W.sub.ash = u.sub.clay.sbsb.3 W.sub.clay + u.sub.Ca.sbsb.3 W.sub.Ca + u.sub.Ti.sbsb.3 W.sub.Ti (8) ______________________________________
The simultaneous solution of these 3 equations yields the amounts of the 3 unknown ash components, W.sub.clay, W.sub.Ca and W.sub.Ti. The total amount of ash can then be determined by summing the basis weights of each of the individual ash components.
To avoid the necessity of providing 3 x-ray tubes and 3 x-ray detectors, the prior art has suggested the use of a single high voltage x-ray tube whose anode voltage is successively incremented to three different voltages, such that the x-ray tube is operated at 1 of 3 different voltages during each scan across the sheet. The x-ray detector signal can then be time-wise demultiplexed to correspond to the 3 different voltages.
The detector and x-ray tube are positioned on directly opposite sides of the sheet and scanned in unison back and forth across the moving sheet so that the sensor is exposed to all portions of the sheet. The directions perpendicular to the direction of motion of the sheet are called the "cross-directions." During each of the 3 scans, the detector signal is integrated. Then, following the 3 scans, the previously mentioned simultaneous equations can be solved to determine the individual amounts of each of the ash components. The individual amounts are then summed to get a value indicative of the total amount of ash.
In many situations, the ash components may be held together in the coating material by a binder, such as latex. Typically, the binder will be mixed with the ash components in some known, fixed proportion. Accordingly, by determining the total amount of ash, one can also indirectly determine the total amount of coating material applied to the sheet.
Unfortunately, however, because of the inherently slow nature of switching a high voltage power supply between different voltage outputs, such a system may practically provide, at best, basis weight and chemical composition measurements only once after every three scans. Thus, such a system will be useless in many situations where it is necessary to achieve rapid, substantially continuous cross-directional control of coating thickness at a plurality of slice positions.
The prior art also discloses the use of a single x-ray tube, wherein the energy of the x-ray beam emanating from the tube is so distributed that the effective absorption coefficients for each of the ash components is effectively equalized. Therefore, the output from such a sensor is directly indicative of the total basis weight of the ash independent of the relative proportions of the various ash components. However, because such a sensor cannot distinguish between each of the 3 ash components, the sensor, of course, cannot provide outputs indicative of the chemical composition of the ash.